A general need exists for chemical sensors which can be used both to identify and to quantitatively measure a wide range of molecular constituents in a gaseous medium. In the past, the predominant techniques have had relatively low structural specificity, for example, electrochemical or calorimetric detection of oxidizable gases (see, for example, M. J. Madou and S. R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, N.Y. (1989)). However, the range of environmental species requiring detection and monitoring has exceeded the capabilities of these instruments. Improved sensors are needed in which the molecular structure of gaseous species giving a positive sensor response can be assigned or confirmed with a high degree of certainty.
A particularly attractive method for detection and identification of unknown molecular species is Raman spectroscopy. Like infrared spectroscopy, the Raman spectrum gives a series of sharp lines which constitute a unique fingerprint of a molecule. These lines correspond to frequencies of molecular vibrations and therefore can be related directly to molecular structure. Raman spectroscopy employs visible light and hence can be used for remote sensing over optical fibers using common visible or near infrared laser sources. In addition, water, which is ubiquitous in most sampling environments, gives only a very weak Raman spectrum and hence would not greatly interfere with detection. Raman is, however, a notoriously insensitive technique requiring nearly neat samples to achieve reasonable signal levels. Raman detection of dilute gas phase species would require an extremely long pathlength, with difficulties of optical alignment and associated ultraclean optics for multiple reflection cells.
When molecules are adsorbed onto some solid substrates, an enhanced Raman signal of the adsorbate is obtained, with up to 10.sup.7 greater signal intensity than would normally be observed. This phenomenon forms the basis of Surface Enhanced Raman Spectroscopy (SERS). Surfaces which give rise to the enhancement are certain metals such as Ag, Au and Cu; most other metals show no such effect. To maximize the effect, the metal must have a high degree of surface roughness. The latter has been obtained by electrochemical oxidation/reduction of metal surfaces, or by vapor deposition of the metal onto a high roughness substrate.
SERS is typically observed with the substrate in aqueous solution. Organic molecules readily displace water from metal surfaces, acting in some cases to preconcentrate molecules present at high dilution. For this reason, SERS has been proposed as a promising technique for detecting low levels of pollutants in sources of drinking water. However, if the roughened metal substrates are placed in air, they do not readily adsorb foreign molecules present in the gas phase, or the surface becomes saturated with atmospheric gases; thus, the same preconcentration is not obtained. It is for this reason that SERS has not been exploited as a gas phase sensor.
A particular example where high specificity and low detection limits are required is in the aerospace industry, which presently uses large quantities of hypergolic fuels consisting of hydrazine and its derivatives. The toxicity, tumorigenic and explosive properties of the hydrazines necessitate their real-time, low level detection in order to ensure the protection and safety of personnel. Threshold limit values of 100 ppb for hydrazine and 200 ppb for monomethylhydrazine requires portable instrumentation and dosimeters capable of detection of these materials at the 10 to 100 ppb level. However, portable instrumentation based on present state-of-the-art sensor technologies are unable to achieve interference free, real-time detection at the parts per billion level. In addition to the low level sensing, the need exists to sense in real time leaks in the 0-200 ppm range that would be encountered during ground based operations of the space shuttle.